The manufacture of aromatic acids useful as monomers typically is a complex, multistep process. For example, 2,6-naphthalenedicarboxylic acid (2,6-NDA) can be manufactured by a five step synthesis process which includes the steps of reacting o-xylene and butadiene in an alkenylation reaction to produce 5-ortho-tolylpentene, cyclizing the 5-ortho-tolylpentene to form 1,5-dimethyltetralin (1,5-DMT), dehydrogenating the 1,5-DMT to produce 1,5-dimethylnaphthalene (1,5-DMN), isomerizing the 1,5-DMN to produce 2,6-dimethylnaphthalene (2,6-DMN), and oxidizing the 2,6-DMN to produce 2,6-NDA.
Crude NDA produced by such a process will contain a wide variety of what are believed to be undesired process-related materials. Many of these materials will be isomers of 2,6-NDA or mono- or trifunctional reaction products. Other undesired process-related materials contained in the crude NDA will be reagents such as catalyst metals carried through the various reactions steps, and color bodies formed during the reaction steps. As used herein, the term "process-related material" means any material that is formed or added in any process step leading up to the manufacture of aromatic acid monomer product, including but not limited to, catalysts, products of side reactions, undesired oxidation products, undesired isomers and the like.
It is believed that in the preparation of polyesters from monomers such as NDA, monomer purity is critical to satisfactorily achieving high molecular weight polymers and a sufficiently fast kinetic rate of polymerization. For this reason, polymer manufacturers typically require that monomer impurities such as mono-functional and tri-functional glycols and carboxylic acids be minimized or eliminated from monomers to be used in polymerization reactions. For example, terephthalic acid and isophthalic acid typically are expected to contain less than 200 parts per million or less by weight total of monocarboxylic and tricarboxylic acids. Similarly, ethylene glycol used in polymerization reactions typically is expected to contain no detectable impurities.
Tricarboxylic acids are thought to be undesirable because such trifunctional compounds can cause undesired cross-linking of polymer chains. Such cross-linking is reported to contribute to slow rates of crystallization and polymer brittleness, both of which are undesired characteristics in many applications. Additionally, when cross-linking becomes substantial, a "gel point" is reached. At this point, the polymer cannot be melt polymerized or melt fabricated and is no longer considered to be a thermoplastic material.
Monocarboxylic acids and other monofunctional materials are believed to be undesirable components in monomers because they act as "chain-stoppers" which inhibit the development of molecular weight and because they decrease reaction kinetics. If the concentration of such materials is too high, the polymerization rate can become zero due to termination of otherwise reactive end-groups.
Color bodies of various types are thought to be undesirable in monomers. The presence of color bodies in monomer can result in substantially greater color in a polymer than would appear likely from seemingly small amounts of color visible in a monomer, thus making even minute amounts of color bodies in monomers undesirable. As used herein, the term "color bodies" refers to any carboxylic acid containing process-related material present in a monomer or polymer that can contribute to the presence of color in the monomer or polymer if present in sufficient amount.
Metals such as entrained catalyst metals also are thought to be undesirable components in monomers. For example, entrained cobalt and manganese oxidation catalyst are believed to be undesirable monomer impurities because it is expected that they may affect the rate of polymerization and polymer color in an unpredictable way. Such metals also are thought to sometimes affect the amount of color visible in a monomer or polymer.
Because it is believed that the presence in monomer of undesired process-related materials such as byproducts, reagents and impurities like color bodies can result in an inferior polymer product, substantial effort typically is devoted to improving the purity of monomers such as 2,6 NDA to provide a quality of product deemed acceptable by customers.
For example, purified aromatic acids have been produced from crude aromatic acids by slurrying the effluent from a crude aromatic acid oxidation process, passing the slurry through a plurality of heaters until the reaction products are dissolved, passing the resulting solution over a purification catalyst, and thereafter crystallizing a purified product. Such a process requires substantial time and energy beyond that expended to produce crude aromatic acid, and therefore substantially increases the cost of the monomer.
Alternatively, high purity monomer can be manufactured by starting with a relatively high purity feedstock, such as a process in which relatively pure 2,6-naphthalenedicarboxylate (2,6-NDC) is hydrolyzed to form relatively pure NDA. This process also is cost intensive because of the complexity and expense of producing the relatively pure NDC feedstock.
What is needed is a cost effective way to produce aromatic acids such as NDA which are suitable for use in polymer applications.